1. Technical Field
The invention relates to a method for measuring the profile a reflective surface of an object, comprising the steps of projection of a defined pattern composed of at least two different light intensities onto the surface to be measured; observation of at least one section of the surface by means of at least one camera, and evaluation of the observed section based on the camera data. The invention furthermore relate to an apparatus for determining the profile and other characteristics of the reflective surface of and object having means for producing a light patter, and having at least one camera for observing at least one section of the surface.
2. Prior Art
A number of scanning methods for measuring the profile of surfaces of flat or curved objects are known from practice, in the case of which methods, for example, scanning elements which can be extended pneumatically are moved into contact with the object to be measured and supply to an arithmetic and logic unit a signal which corresponds to the movement carried out, which arithmetic and logic unit uses this signal to determine the local separation and, based on a large number of such measurements, to make a statement on the profile of the surface of the object. The use of such measurement methods and the associated apparatuses is complex since, bearing in mind the sensitivity of the object to be measured, the measurement probe can be extended only slowly, may possibly damage the object or its surface (for example scratching it or making an indentation) and, furthermore, the temperature range in which such techniques can be used is extremely limited. Such tactile measurement methods are thus generally inferior to non-contact measurement methods.
The article by Kxc3x6rner, K. et al., xe2x80x9cSchnelle Planitxc3xa4tsmessung von groxcex2flxc3xa4chige Objektenxe2x80x9d [Rapid planarity measurement of large-area objects], MSR-Magazine, Nov. 12, 1995, pages 16-18 describes an optical measurement method for measuring the planarity of thin glass. In this case, based on a light source a line grid is imaged sharply on a thin-glass plate, with the optical axis at a large angle (84xc2x0) to the normal to the thin-glass plate. This image is observed from an angle which is also large using a CCD line-scan camera, from whose observation information about the height and/or about the planarity of the thin glass is calculated by means of phase-evaluating algorithms. A first imaging stage is provided between the line grid and the thin-glass plate in the optical direction of incidence, and a second imaging stage is provided between the thin-glass plate and the line-scan camera in the optical direction of reflection. The first imaging stage images the line pattern of the line grid on the front face of the glass, while the second imaging stage in turn produces a sharp representation of the image on the line-scan camera. The known technique has a number of inadequacies: on the one hand, the geometric arrangement of the components is complex since the optical axes from the imaging stages strike the thin-plate glass at a very flat angle. The light emerging from the line grid is imaged via the first imaging stage on the glass plate; the line grid itself is inclined with respect to the optical axis, so that the distances between the lines turn out to be different. If minor changes are made to the position of the thin-glass plate which is currently being measured and thus cannot be assumed to be constant, the entire device must be readjusted. However, the calculated values are subject, at the least, to a severe error tolerance. Since even very small discrepancies in the planarity to be measured of an ideal-planar surface lead to severe distortion of the image of the line grid, the resolution of the apparatus is limited. Furthermore, such an apparatus also occupies a large amount of space, so that high space costs are incurred when such an apparatus is used for on-line measurement. Space problems may occur in particular after such apparatuses have been re-equipped. Finally, it should also be mentioned that the line-scan camera is very sensitive to stray light, as a result of which costly screening must be provided; this is due to the fact that the image is produced sharply on the camera, with the camera being arranged at an angle to the glass surface, since the measurement point would otherwise be outside the field of view, so that the camera can also be influenced by other light (and variations in it). In order to evaluate the measurements, the imaging conditions must be known and must be taken into account. If one wishes to carry out a three-dimensional measurement using the known technique, this can be done only by moving the glass plate relative to the image section which the camera is observing, and carrying out a number of measurements successively. The technique is not designed to carry out measurements at a number of points on the surface at the same time, since a sharp image of the line grid is produced in only one area. In consequence, exact measurements take a time in the order of magnitude of one to several minutes. The imaged pattern can no longer be evaluated for measurement of surfaces with relatively large radii.
U.S. Pat. No. 5,110,200 describes a method and an apparatus for observing and for measuring the human cornea. A video image of a reflection of an illuminated ring is in this case examined for discontinuities in the brightness of the image, in order to determine the contour of the cornea. However, this method is not suitable for moving objects, such as moving conveyor belts, since evaluation is impossible when a large number of changes take place at the same time. Furthermore, the method is likewise not suitable for evaluating objects which reflect light a number of times, as is the case for the two surfaces of a glass pane. The known method can also not determine discrepancies in the planarity of a surface. Finally, the known method is very time-consuming, so that it is not suitable for industrial use.
DE-A-44 01 541 likewise describes a method for determining the surface structure of the cornea. In this case, a camera which is arranged behind a perforated shutter detects the light which is emitted from a light-emitting diode (which moves on circular paths with variable radii) and is reflected from the surface of the retina. Local defects in the retina are detected from discrepancies in the circular shape of the reflected light. Where reflections merge into one another or are interrupted, the observation can be carried out intermittently over a period of time to allow these reflections to be associated with the respective output radius of the light-emitting diode. The known method is time-consuming and can thus not be used on an industrial scale. It is also not suitable for double-reflecting materials. Furthermore, it is impossible to superimpose a movement of an object to be measured on the movement of the light source.
The invention is based on the object of providing a technique (method and apparatus, respectively), for measuring the profile of a reflective surface of an object which is of simple design and can be used with precision.
This object is achieved according to the specification, drawings and claims herein in that the pattern produces a mirror image in the reflective surface, and in that the observed section comprises a section of the mirror image of the pattern, and according to the description of the apparatus in that the camera being set such that it is focused on a mirror image of the light pattern:
The technique according to the invention makes it possible, in particular, to measure the profile of a reflective surface of an object which is composed of a material which is at least partially transparent for light at specific wavelengths and which reflects this light on at least one further surface arranged behind the surface. Examples of such objects are glass plates, plastic sheets, reflective surfaces covered with a transparent coating, laminated car window panes, etc. The mutually superimposed reflections from the at least two surfaces are advantageously separated, so that measurements are obtained with a precision not achieved before.
It is not only possible for the camera to observe the mirror image directly, but also for the camera to observe the mirror image indirectly via a mirror arrangement.
Expediently, in the former case, the optical axes (planes) then include an angle which is less than 90xc2x0, preferably (very) much less, between the pattern and the mirror image on the one hand, and between the mirror image and the camera on the other hand. The two optical axes are preferably arranged such that they include the same angle on both sides of the normal to a planar surface as the angle of incidence and the angle of reflection of light with respect to said normal, with the total of these angles giving the included angle. It is possible and preferable for this included angle to be very small. The aperture angle between the pattern and camera is the angle (on the planar surface) between those planes on which the line-scan camera and the observed mirror image section on the one hand, and the light pattern and the observed section on the other hand, are arranged. In the case of a matrix camera, the corresponding angles are preferably close to one another for each line.
In the second case, in which a mirror arrangement is provided, the latter preferably comprises a parabolic mirror which always images the observed mirror image sharply, irrespective of the distance between the reflective surface and the parabolic mirror, at a point at which the camera is then arranged.
A pattern which is easy to produce has equidistant, alternately bright and dark light strips; however, other geometrically defined alternating light/dark sequences are also suitable for use as a pattern, for example those having strips which have more than two different light intensities, checkerboard patterns, and cross-hatched patterns. One particularly advantageous pattern comprises dark (light) strips which cross one another at right angles and enclose light (dark) squares whose edge length corresponds to the width of the strips.
According to the invention, the pattern is reflected directly in the reflective surface. Planarity faults (that is to say small height changes) in the measured two-dimensional or three-dimensional reflective surface cause distortion in the mirror image, with a minor projection or indentation in the surface inducing a broader or, respectively, a narrower mirror image in that the incident light is somewhat more strongly compressed or scattered. Using the, camera, it is possible to detect the intensity change (and/or the profile) in the light/dark mirror image precisely for each point in the mirror image, and thus to evaluate even very small changes in the degree of brightness, and, for example by forming the differences to an ideal mirror image, to draw conclusions about variations in the surface to be measured. The local inclination differences are calculated on the basis of boundary changes in the dimensions of the light/dark intervals observed by the camera. The simple design of the apparatus according to the invention and the compact and direct method according to the invention allow the surface profile to be recorded extremely quickly and reliably. The light originating from the pattern is parallel when it arrives at the surface to be measured. The light/dark sequence of the pattern is reflected in the surface. If the surface were completely planar, this would accordingly result in a mirror image that is exactly proportional to the light/dark patternxe2x80x94provided the lateral offset from the normal is not too large. Since the design of the mirror image (distances etc.) is known, the arrangement according to the invention advantageously makes it possible for the discrepancies from an ideal image to be calculated with respect to the known dimensions of the line grid rather than in comparison with a recorded image of a normal surface, as a result of which the accuracy of the evaluation is improved. The amount of distortion of a strip in the observed mirror image is measured, and this is used to determine its inclination with respect to a planar surface, with the capability to express the inclination as an angle. The inclination of the corresponding area element in the mirror image can thus be determined for each strip in the pattern; if these elements are arranged in a row (integration), starting from a defined point (for example as a result of contact) on the surface, information is also obtained about the successive height rise occurring as a result of the inclinations, and the profile can thus be xe2x80x9cfollowedxe2x80x9d.